In the continuous casting method of manufacturing steel, molten (liquid) steel is cast directly into thin strip by a casting machine. The shape of the strip is determined by the mold of the casting machine, which receives the molten metal from a tundish and casts the metal into thin strip. The strip may be further subjected to cooling and processing upon exit from the casting rolls.
In a twin roll caster, molten metal is introduced between a pair of counter-rotated horizontal casting rolls which are internally cooled so that metal shells solidify on the moving casting roll surfaces, and are brought together at the nip between the casting rolls to produce a thin cast strip product, delivered downwardly from the nip between the casting rolls. The term “nip” is used herein to refer to the general region at which the casting rolls are closest together. The molten metal may be poured from a ladle through a metal delivery system comprised of a tundish and a core nozzle located above the nip, to form a casting pool of molten metal supported on the casting surfaces of the rolls above the nip and extending along the length of the nip. This casting pool is usually confined between refractory side plates or dams held in sliding engagement with the end surfaces of the casting rolls so as to restrain the two ends of the casting pool.
When casting steel strip in a twin roll caster, the thin cast strip leaves the nip at very high temperatures, of the order of 1400° C. If exposed to normal atmosphere, it will suffer very rapid scaling due to oxidation at such high temperatures. A sealed enclosure that contains an atmosphere that inhibits oxidation of the strip is therefore provided beneath the casting rolls to receive the thin cast strip, and through which the strip passes away from the strip caster. The oxidation inhibiting atmosphere may be created by injecting a non-oxidizing gas, for example, an inert gas such as argon or nitrogen, or combustion exhaust reducing gases. Alternatively, the enclosure may be substantially sealed against ingress of an ambient oxygen-containing atmosphere during operation of the strip caster, and the oxygen content of the atmosphere within the enclosure reduced during an initial phase of casting, by allowing oxidation of the strip to extract oxygen from the sealed enclosure as disclosed in U.S. Pat. Nos. 5,762,126 and 5,960,855.
The length of a casting campaign of a twin roll caster has been generally determined in the past by the wear cycle on the core nozzle, tundish and side dams. Multi-ladle sequences can be continued so long as the source of hot metal supplies ladles of molten steel, by use of a turret on which multiple ladles of molten metal can be transferred to operating position. Therefore, the focus of attention in the casting has been extending the life cycle of the core nozzle, tundish and side dams, and thereby reducing the cost per ton of casting thin strip. When a nozzle, tundish or side dam would wear to the point that one of them had to be replaced, the casting campaign would have to be stopped, and the worn out component replaced. This would generally require removing other unworn components as well since otherwise the length of the next campaign would be limited by the remaining useful life of the worn but not replaced refractory components, with attendant waste of useful life of refractories and increased cost of casting steel. Further, all of the refractory components, both replaced and continued components, would have to be preheated the same as starting the original casting campaign before the next casting could be done. Graphitized alumina, boron nitride and boron nitride-zirconia composites are examples of suitable refractory materials for the side dams, tundish and core nozzle components. Also, since the core nozzle, tundish and side dams all have to be preheated to very high temperatures approaching that of the molten steel to withstand contact with the molten steel over long periods, considerable waste of casting time between campaigns resulted. See U.S. Pat. Nos. 5,184,668 and 5,277,243.
Also, the side dams wear independently of the core nozzles and tundish, and independently of each other. The side dams must initially be urged against the ends of the casting rolls under applied forces, and “bedded in” by wear so as to ensure adequate seating against outflow of molten steel from the casting pool. The forces applied to the side dams may be reduced after an initial bedding-in period, but will always be such that there is significant wear of the side dams throughout the casting operation. For this reason, the core nozzle and tundish components in the metal delivery system could have a longer life than the side dams, and could normally continue to be operated through several more ladles of molten steel supplied in a campaign if the useful life of the side dams could be extended. The tundish and core nozzle components, which still have useful life, are often changed when the side dams are changed to increase casting capacity of the caster. Further, the core nozzle must be put in place before the tundish, and conversely the tundish must be removed before core nozzle can be replaced, and both of these refractory components wear independently of each other.
In addition, no matter which refractory component wears out first, a casting run will need to be terminated to replace the worn out component. Since the cost of thin cast strip production is directly related to the length of the casting time, unworn components in the metal delivery system are generally replaced before the end of their useful life is a precaution to avoid further disruption of the next casting campaign. This results in attendant waste of useful life of refractory components.
Each side dam is generally held in place during casting by a side dam holder. The side dam typically includes a V-shaped beveled bottom portion and the side dam holder typically includes a V-shaped receptacle into which the V-shaped beveled bottom portion of the side dam is seated. The V-shape configuration serves to position and hold the side dam in place during casting. However, such side dam assemblies limit the useful life of the side dams before adversely impacting the edges of the cast strip and risking serious damage to the casting equipment. Specifically, the worn side dams and side dam holders may allow bleeding molten metal if the side dams are allowed to wear past a certain point, and result in damage to the casting equipment. Therefore, the side dams are usually replaced before such damage to the edges of the cast strip and to casting equipment can occur limiting the duration of the casting campaign. As explained above, when the side dams are changed, the removable tundish and nozzle core will generally also be changed and a new casting campaign started. The casting costs per ton of thin strip cast thus could be considerably reduced if the useful life of the side dams could be extended.
Further limitations and disadvantages of previously used and proposed thin strip casting systems and methods will become apparent to one of skill in the art, through comparison of such systems and methods with the present invention as set forth in this present application.